The procedure of monitoring and analyzing uterine contractions, during both pregnancy and labor, yields significant information concerning the condition of the fetus as well as the advancement of labor. Such a procedure is useful both during routine pregnancies and especially during difficult pregnancies, those which have increased risk to the health of the fetus, to systematically evaluate fetal stress. The procedure is also used when labor is induced. Information indicating fetal distress during pregnancy, labor, and delivery will prompt remedial action, including caesarean delivery, which may save the fetus from harm and even death. Thus, contraction frequency, duration, intensity, and resting tone are now monitored as part of accepted, standard, obstetrical procedure.
The fetal monitors widely used to monitor the uterine activity of pregnant women, as well as the condition of the fetus while in the uterus, are typically quite sophisticated. Examples of currently available fetal monitors include the FetaScan from International Biomedics, Inc.; the Corometrics 115; and the Hewlett-Packard 8040A. Regardless of their sophistication, however, fetal monitors require a device or element to actually sense the uterine contractions.
One example of such a sensing element is a tocodynamometer or tocotransducer. Tocodynamometers can sense uterine activity externally and non-invasively. The advantages offered by those devices have caused them to be widely used with fetal monitors. Tocodynamometers measure the hardness of the abdominal wall, which is an indication of uterine activity, through various mechanical elements. The tocodynamometer is held adjacent to the patient's abdomen, usually by a belt-like device, in the vicinity of the fundus (the top of the uterus). The tocodynamometer is initialized by setting the recording level so that it is about 10-15 mmHg between contractions. The output of the device is transmitted to the fetal monitor through a pressure transducer, the transducer converting the pressure change information received from the tocodynamometer to an electrical signal which it delivers to the fetal monitor.
Externally applied devices like tocodynamometers can often provide sufficient information to enable a physician to treat the mother and child during labor and delivery. Such devices may suffer large measurement errors in some cases, however, particularly when the environment has extraneous noise or the mother moves extensively. Therefore, a physician may want to have more accurate measurements than can be obtained using external monitoring devices--especially in those childbirth cases involving a risk of complication.
To obtain more reliable and accurate information about the mother's uterine contractions, a physician will often invoke intrauterine pressure monitoring. Intrauterine devices provide information about the frequency, duration, intensity, and resting tone of the uterine contractions. More important, intrauterine devices reduce measurement errors relative to external devices because the uterine pressure is measured directly.
One of the most widely used intrauterine devices is an open-ended, liquid-filled catheter. The catheter is inserted into the uterus so that the force of the uterine contractions can be transmitted through the uterine fluids and the sterile liquid solution in the catheter to an external pressure-measuring element such as a pressure transducer, strain gauge, or the like. A cumbersome procedure is required to fill the catheter with liquid and then to couple the catheter to the pressure-measuring element to complete a liquid path or column from the uterus to the transducer. A relatively long length of liquid-filled tubing must travel over the mother to the pressure-measuring element. The pressure-measuring element is connected to a monitor. Typical monitor devices include cathode ray tube displays, digital displays, recording machines, printers, and plotters.
In addition to the proper set-up of the liquid-filled catheter, the catheter must be primed with a sterile solution so that any air bubbles within the catheter are removed and a continuous liquid column is provided from the external pressure-measuring element to the tip of the catheter within the uterus. One of the disadvantages of the liquid-filled catheter is the time required to fill and prime the catheter. Particularly in critical situations, this procedure wastes valuable time. In addition, an air bubble or biologic debris will enter the open end of the catheter occasionally and compromise pressure measurements. This necessitates another time-consuming procedure: the catheter must be flushed with sterile solution to remove the air bubble or debris. Sometimes, the catheter must be replaced altogether.
Recently, disposable intrauterine pressure catheters have been introduced which place a miniature transducer in the catheter tip. The product packaging for the competitive products of Utah Medical Products, Inc. (the INTRAN PLUS.TM. device), Hewlett-Packard Company (the HP 13995A device), and Corometrics Medical Systems, Inc. (the SENSORTIP.TM. device) all describe such devices. Typically, most of these devices have relatively large tips and are supported by stiffening inserts which may damage the fetus and uterine walls. In addition, the insertion of the large tip may push bacteria high into the uterus where it is more likely to cause infection.
U.S. Pat. No. 4,944,307 issued to Hon et al. discloses an open-ended, liquid-filled catheter with an external pressure transducer. The patent criticizes transducer-tipped intrauterine pressure catheters as costly and a departure from the tried and true apparatus for measuring contractions. Nevertheless, the '307 patent recognizes the need to zero the device while in place in the uterus and provides structure to do so. Such structure may enable an open-ended, liquid-filled catheter to be zeroed during use; it is inapplicable for a transducer-tipped intrauterine pressure catheter.
U.S. Pat. No. 4,966,161 issued to Wallace et al. also discloses a catheter (although not liquid-filled) with an external pressure transducer. The transducer is located in a connector which includes a slide valve used to zero the transducer and monitor. The backside (non-uterus) of the transducer diaphragm is continuously vented to atmospheric pressure. With the valve in the "monitor" position, the transducer communicates with the fluid in the uterus through a lumen in the catheter and the uterus side of the transducer diaphragm "sees" the uterine pressure. With the valve in the "zero" position, however, a barrier prevents fluid communication between the uterus and the transducer while the uterus side of the transducer diaphragm is vented to atmospheric pressure. Because both sides of the transducer diaphragm "see" atmospheric pressure, any offset caused by the electric components of the transducer can be detected and corrected by the monitor. The structure used to zero the transducer and monitor is positioned on the disposable connector to which the catheter is attached.
As stated above, the Hewlett-Packard Company markets a transducer-tipped intrauterine pressure catheter (the HP 13995A device). It may be gathered from the instructions for use published by Hewlett-Packard for the device that the zero setting is realized before introducing the catheter as long as its measuring tip is exposed to ambient atmospheric pressure. The instructions explain that renewed zero setting is not required once the measuring tip is in situ. In fact, the instructions for Hewlett-Packard's 13995A intrauterine pressure catheter state: "Caution: Do not rezero the monitor once the catheter is inserted." Thus, the device does not provide any way to zero either the monitor or the pressure transducer after insertion (although the device can be zeroed before use).
The assertion that renewed zero setting is not required once the measuring tip is in situ is true only as long as the system does not experience any disturbances. If the monitor should fail during a measurement, which may last several hours, then a new zero setting is indispensable. Failure may occur, for example, when the main cable of the monitor is inadvertently disconnected when connecting another instrument to the monitor. Accordingly, it is desirable to provide the system with the ability to zero the system in situ.
U.S. Pat. No. 4,901,735 issued to von Berg discloses a transducer-tipped intrauterine pressure catheter which admits a reference pressure to the tip to zero the transducer while in situ. The sensor tip is surrounded by an inflatable balloon which, when deflated, acts as a diaphragm over the sensor tip pressure measuring connection. Thus, the balloon covers the transducer connection which measures uterine pressure and acts as a pressure-transmitting membrane. The reference connection of the transducer is exposed to atmospheric pressure via an air passage. To zero the transducer, a syringe filled with air forces air into the air passage, thereby inflating the balloon. Both sides of the transducer, surrounded by the air-filled balloon, now "see" the uniform air pressure prevailing inside the balloon. Therefore, the monitor should show a relative pressure of zero value. When the syringe is removed, the pressure in the balloon drops until it again engages the surface of the sensor and acts as a pressure-transmitting membrane.
The device disclosed by the '735 patent relies heavily on the loose-fitting balloon to function as a pressure-transmitting membrane. Moreover, additional, complex structures including the balloon, a syringe, and a syringe coupling are required to zero the device. All of these structures are positioned on the disposable catheter. Finally, the risk that contaminated air might get into the uterus also arises with such a device.
As stated above, Corometrics Medical Systems, Inc. markets a transducer-tipped intrauterine pressure catheter (the SENSORTIP.TM. device). It may be gathered from the instructions for use published by Corometrics for the device that the zero setting is realized in a manner similar to that disclosed in the '735 patent. The user is provided with a "1 cc zeroing syringe" and Corometrics' device includes a balloon and a syringe coupling (or "zero port"). To zero or rezero at any time that verification of the baseline is necessary, the user is instructed to fill the syringe with air, attach the syringe to the syringe coupling, and inject the air into the syringe coupling. The user must verify that the balloon remains inflated then zero the monitor after allowing it to stabilize. Having zeroed the device, the user is instructed to remove the syringe.
The '735 patent also suggests a second way to zero the transducer in situ. The output of the pressure transducer in situ can be compared with a representative reference value. The patent suggests that a computer might calculate a simulated reference pressure value. The reference pressure value may then be compared for zero setting with the actual pressure value measured.
As stated above, Utah Medical Products, Inc. markets a transducer-tipped intrauterine pressure catheter (the INTRAN PLUS.TM. device). The connector on the end of the disposable catheter has a zero slide valve with an "open" and a "closed" position. When the valve is in the "closed" position, the signal from the pressure transducer travels to the monitor and produces a reading. When the valve is in the "open" position, the signal is cut so that the monitor "sees" a null or zero output voltage. The monitor can then be adjusted, if necessary, to produce a zero reading.
U.S. Pat. No. 4,873,986 issued to Wallace discloses a transducer-tipped intrauterine pressure catheter. (The application which issued as the '986 patent was a continuation-in-part of the application which issued as U.S. Pat. No. 4,785,822; the '822 patent, in turn, refers to U.S. Pat. No. 4,610,256.) The '986 patent addresses the problem of zero balancing the transducer to ensure that the "static" readings it produces are accurate. More specifically, the '986 patent states that the device does not require rezeroing or balancing after the transducer has been inserted inside the uterus. Thus, in situ zeroing is not contemplated. The transducer has a vent channel on the backside of the diaphragm to atmospheric pressure. The opposite (uterus) side of the diaphragm "sees" the pressure in the uterus. The '986 patent discloses that the vent channel can be used to calibrate (distinguish zero) the semiconductor transducer directly using a known pressure source, even when the transducer is inside the uterus.
It is possible to zero a transducer-tipped intrauterine catheter system after insertion simply by removing the catheter from the uterus, thereby exposing both sides of the transducer diaphragm to atmospheric pressure. This introduces another inconvenient and time-consuming procedural step, however, and is undesirable because the catheter tip would no longer be sterile--inceasing the risk of infection.
To overcome the shortcomings of the existing devices for sensing uterine activity and transmitting information to a fetal monitor, an improved intrauterine pressure catheter system is provided. The general object of the present invention is to make the physician's job easier while meeting the patient's needs. It is another object to provide an apparatus for intracavity pressure monitoring that does not require a liquid column to couple the intracavity pressure to a pressure transducer. A related object is to avoid the complexity of running a liquid-filled tube from the catheter to an external pressure transducer. An important object of the present invention is, therefore, to provide an intracavity pressure device in which the pressure transducer is placed directly in the body cavity. Still another object of the present invention is to provide for continuous, accurate measurement of intrauterine contraction pressures during labor and delivery.
Yet another object of the present invention is to increase patient comfort by, among other things, reducing the number of adjustments necessary to assure optimum performance of the monitoring system. A related object is to provide an improved intrauterine pressure catheter system which has a catheter tip profile designed for patient comfort, easy insertion, and assured positioning without slippage. Another object is to provide a system that reduces the risk of infection or contamination during use.
Still another object of the present invention is to permit the monitor to be conveniently zeroed whenever it is desired to do so. The ability to test the electronic components of the system is another object. Yet another object of this invention is to provide an intrauterine device which allows fluid samples to be withdrawn from, and fluids to be infused into, the uterus.